The worldwide consumption of plastics is around 400 billion pounds a year—with only about 12% being recycled. There has been a movement to increase the level of plastics recycling to reduce landfill usage. While municipal waste has traditionally been the main target for recycling, other type of plastic waste such as those generated by industry is gaining more attention. Some industries, such as the biopharmaceutical sector, are more sensitive to the life cycle of their waste. In this market, recyclability gains even more importance for the products that are designed for one-time use.
The majority of the recycled plastics are single layer bottles or films made from PET, HDPE, PVC or LDPE. Mixed plastic waste is generally separated into the different chemical species due to a lack of compatibility between many of these materials—resulting in poor blend properties if blended. In general, recycling of materials containing barrier layers such as epoxies, ethylene vinyl alcohol (EVOH) or polyamides is not practiced, mainly due to compatibility issues.
In addition to the single-layer polymer products, many multi-layer polymer articles exist. The use of multiple layers allows one to take advantage of the properties of each polymer—such as chemical resistance, permeation resistance, weatherability, and physical properties.
It is critical to performance that the various layers of a multi-layer article adhere to one another. This adherence can occur in many ways, including: compatibility or miscibility of the polymers, with the multi-layer formation process generally involving heat and/or pressure; the use of a tie layer (reactive or non-reactive) or adhesive; a surface treatment of one or both surfaces to increase interactions, such as by corona treatment, plasma treatment, chemical etching or even physical abrasion; or by adding functionality into one or both layers that can react covalently with the other layer under processing conditions.
There is a trend in the pharmaceutical industry to reduce the price of the medicine, move towards individualized drugs and reduce the time for new drug development. Moreover, the FDA is enhancing and modernizing the drug production regulations with a push towards improved risk management. The general outcome of these inputs is a move towards adoption of single use manufacturing systems. These systems are typically made completely from plastics and designed to replace stainless steel vessels, pipes and components for production of drugs and batch processed medicines. Currently, several companies manufacture systems with capacities from 1 L to 10,000 L for cell growth, buffer preparation and drug synthesis. These systems typically include bags, tubes, fittings, stirrers and other components. Each of these articles has specific requirements that would necessitate a multilayer structure in the design of some components. It is estimated that several million pounds of plastics will be used in these systems in the next several yeas. The pharmaceutical industry is very particular about the life cycle of their products and is very interested in minimizing the landfill and/or incineration of these single use systems. Moreover, some European regulation bodies are putting emphasis on the life cycle of these disposable systems. Therefore, there is a need to have a manufacturing system that would perform the required functional tasks and be recyclable after it is discarded. Currently the disposable manufacturing systems are incinerated.
Recycling of a multi-layer material (article or scrap) presents special issues, since the layers cannot be easily separated before recycling. First, the whole multi-layer article must be designed to meet specific requirements of high purity, and resistance to permeation of gases and liquids. In the biopharmaceutical industry there is also a need for a surface that resists protein adhesion. Then, in addition, the article must also be capable of being melt blended into a usable article. The polymers in a recyclable multi-layer material must be compatible at both a macro-level and a micro-level.
Compatibility in the polymer adhesion context is represented by low interfacial tension. Specifically the polar component of the surface tension plays a major role in the peeling strength and the work of adhesion between polymers. Typically, the greater the polarity difference, the greater will be the interfacial tension. In addition to the interfacial tension, other factors such as processing parameters and tooling design can affect the peeling strength.
On a macro-compatibility level, the layers of the multi-layer structure need to be compatible only on their surfaces. However, during recycling, the polymers are melt blended in the bulk and must also be compatible at a micro level when the individual polymer chains contact each other. In polymer blends, the important mechanical, transport and optical properties depend on the size of the polymer phases, which in turn is dictated by viscosity ratio, mixing intensity in the extruders and compatibility of the components. In many instances, interfacial area between the phases is the weak point in the polymer bland. In these cases, the failure starts from this area and progresses through the bulk of the material. Strength of this interface, to a great extent is determined by the compatibility of the polymers in the blend. Therefore, a compatible blend would have good surface and processing aspect, reduced phase size, strong domain interface and good melt behavior. For polymer blends compatibility is represented by Flory interaction parameter. It can be shown that Flory interaction parameter and interfacial tension are related to each other.
Many multilayer films do not produce compatible polymer blends. These demonstrate chemical bonding or compatibility at the interface, to allow production of a multilayer film structure, but not micro-scale compatibility that is good enough for an intimate blend with proper morphology and phase size. In multilayer films in which surfaces of the polymers are modified for adhesion, the bulk of the layers would remain incompatible—resulting in a poor recycled blend.
In multilayer films using a tie layer or adhesive, the tie layer may act as a compatibilizer. However, the amount of tie layer generally is not sufficient to provide total surface coverage of the minor phase. Often the tie layer simply forms a separate phase inside the most compatible polymer in the blend—resulting in large incompatible polymer domains.
Functionalized or cross-linked layers may negatively affect viscosity of the blend, making it difficult to reprocess. Ideally, recyclable multilayer films are those in which all of the layers are reasonably compatible with each other. The recycle of many multi-layer films containing only low-performance plastics is generally not worth the recycling cost, and may not produce melt-blends from which useful articles can be formed.
There is a need for multilayer structures designed to have a) excellent physical, chemical, purity, permeation resistance and purity useful in high purity applications, b) layers that adhere well together in the application, and c) are capable of being recycled into useful articles—where the layers that are compatible in a melt blend and the blend has good physical and processing properties.
Compatibility is especially problematic when at least one layer of a multilayer structure is a fluoropolymer. Fluoropolymers, by their nature, are incompatible with, and difficult to adhere to most substances.
Surprisingly, Applicant has now developed multilayer structures in which the layers adhere well, have the excellent properties for use in high purity applications, and are capable of being recycled. One additional advantage of recycling the multilayer structures of the invention is that fluoropolymers, and other polymers used in these types of structures, can be relatively expensive materials, and the articles formed from the recycled blend can receive a performance benefit from the special properties of the fluoropolymer and other high-performance recycled polymer layers.